The present invention relates to a device for detecting the position of an apical foramen of a tooth root for use in dental treatment and more particularly to a method for achieving zero-adjustment of the apical foramen position detector.
In dental treatment, it is usual to treat any vital tooth without extracting it by enlarging, cleaning and disinfecting its root canal and then by filling the root canal with a filling. For a root canal treatment it is necessary to detect accurately the position of the end of the root canal (hereinafter referred to as "an apical foramen").
A device for electrically detecting the apical foramen's position is disclosed in the Japanese laid open patent publication No. 174144/85. The device is such that two AC voltages having different frequencies, for example of 1KHz and 5KHz, are applied to an electrode abutting on oral mucosa and a measuring needle inserted into the root canal of a tooth. The two AC voltages of different frequencies are generated by a pulse generator with a standard frequency preset to 1KHz, and its output is supplied to a measuring needle and an electrode respectively. A resistor is connected between the electrode and the pulse generator and both terminals of the resistor are connected to respective amplifiers. One of the amplifiers is connected at its output to a filter for selecting 1KHz, the output of which is rectified through a rectifier circuit and connected to one of the input terminals of a comparator. The output of the other amplifier is connected to a filter for selecting 5KHz, the output of which is connected through a rectifier circuit to the other input terminal of the comparator. The comparator's output is connected to an amplifier, the output of which is then connected to a display unit. When a dentist slowly inserts the measuring needle into the root canal of a patient's tooth, an impulse signal of 1KHz and an impulse signal of 5KHz, generated by the pulse generator, are multiplexed within a specified period and released. In response to the output signal and the impedance between the measuring needle and the electrode, a weak current is generated which flows through the resistor and is amplified by the amplifiers respectively.
While only a weak current responding to the impulse signal of 1KHz passes through one filter, only a weak current responding to the impulse signal of 5KHz passes through the other filter. While the measuring needle is being moved in the tooth's root canal, the impedance between the measuring needle and the electrode is kept substantially constant, thereby each voltage responding to an input signal's frequency, i.e. an output signal through one of the filters is kept substantially constant. When the measuring needle is inserted further and is nearly out of the root canal's end, the impedance between the measuring needle and the electrode decreases to correspondingly increase the two output voltages of the corresponding filters in such a way that one output voltage changes slowly and the other changes quickly depending upon their frequency responses.
Attention is paid to the difference between both output voltages. Although the output voltages vary at different rates due to their different responses to the input frequencies, their changing ratios increase to the maximum when the measuring needle has reached the apical foramen. Accordingly, the position of the apical foramen can be detected as the point whereat the minimum difference of the output voltages is obtained.
However, to correctly determine the above-mentioned minimum point it is irrevocably necessary to advance the needle tip beyond the apical foramen's position. The advancement of the measuring needle past the root's apex is undesirable and may cause the over-penetration of the needle through the tooth's root canal.